Regulation of vascular contraction and, thus, vascular resistance is generally associated with the activation of receptors coupled to guanine nucleotide associated proteins (G-proteins). Tyrosine kinase(s), enzymes critically important to vascular smooth muscle cell growth, may participate in the dynamic process of controlling vascular tone; a particularly important tyrosine kinase-dependent pathway is the mitogen activated protein kinase or MAPK pathway. The unifying hypothesis of this project is that G-protein coupled agonists, specifically 5-HT (serotonin, 5-hydroxytryptamine), stimulate vascular tyrosine kinase(s) to produce contraction. This is a novel and undescribed signal transduction pathway for the G-protein coupled agonist 5-HT; importantly, contractility to 5-HT and other agonists is dramatically increased in diseased states with documented vascular growth and remodeling (hypertension, atherosclerosis, restenosis) and contractile response to 5-HT is an excellent marker for vascular disease. Thus, tyrosine kinase(s), as activated by 5-HT, may play a significant role in the altered function of the vasculature in disease. Two specific experimental hypotheses which integrate studies from protein to the whole animal will be addressed: 1) 5-HT activates tyrosine kinase(s), in particular the tyrosine kinase MAPK kinase (MEK), to result in direct vascular smooth muscle contraction and potentiation of vasoconstrictor- induced contraction, 2) 5-HT increases tyrosine kinase activity, as measured by protein tyrosyl-phosphorylation and MEK activity, in vascular smooth muscle cells and arteries. Four experimental approaches will be employed: 1) isolated tissue bath technique to measure vascular contractility; 2) myograph technique to measure resistance vessel contractility; 3) Western analyses of vascular smooth muscle cell lysates and arteries to determine 5-HT-induced changes in protein tyrosyl-phosphorylation, especially that of the substrates of MEK (the MAPKs); 4) MEK activity assays. Based on preliminary results, it is predicted that tyrosine kinases, including MEK, mediate a portion of 5- HT-induced contraction, both direct contraction and that which potentiates responses to other cardiovascular hormones. The results of these experiments will reveal novel mechanisms for 5-HT signal transduction in vascular smooth muscle contraction. The pathways established will set the basis for a novel manner of normal cellular activation by 5-HT and most likely other G-protein coupled agonists, and can thus be an initial step in understanding how signal transduction for 5-HT and other agonists may change in diseased states associated with vascular smooth muscle growth or remodeling.